This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
In such facsimile systems, it is frequently desirable to generate handshake signals which are transmitted from the receiver back to the transmitter so as to advise the transmitter of the readiness and capability of the receiver to continue to receive signals. In systems which do not utilize handshake signals, the communications network connection could be broken but the operator at the transmitter would not become aware of the broken connection until the entire document at the transmitter has been scanned. This could result in a considerable consumption of time for an operator at the transmitter or at the receiver. In other situations, it may be desirable for a transmission to be interrupted at the receiver. The transmitter can be advised of this interruption when it fails to receive the anticipated handshake signal.
Heretofore, handshaking in facsimile transmissions has required the use of a portion of the available bandwidth to provide the handshake function. In systems of this type, the facsimile transmitter utilizes a portion of the available bandwidth for the facsimile information transmission while another portion of the available bandwidth is utilized for the handshake signals. Such a full duplex operation is, under some circumstances, considered undesirable since the use of a portion of the available bandwidth for handshake signaling necessarily decreases the rate of transmission of facsimile information through the remaining available bandwidth and can result therefore in increased transmission times. Where a channel is conditioned for a full duplex operation, channel quality for facsimile operation may be adversely affected.